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In perceiving 3D shape from ambiguous shading patterns, humans use the prior knowledge that the light is located above their head and slightly to the left. Although this observation has fascinated scientists and artists for a long time, the neural basis of this "light from above left" preference for the interpretation of 3D shape remains largely unexplored. Combining behavioral and functional MRI measurements coupled with multivoxel pattern analysis, we show that activations in early visual areas predict best the light source direction irrespective of the perceived shape, but activations in higher occipitotemporal and parietal areas predict better the perceived 3D shape irrespective of the light direction. These findings demonstrate that illumination is processed earlier than the representation of 3D shape in the visual system. In contrast to previous suggestions, we propose that prior knowledge about illumination is processed in a bottom-up manner and influences the interpretation of 3D structure at higher stages of processing.functional MRI | multi-voxel pattern analysis | shape-from-shading | visual cortex | bottom-up processing A lthough the perception of 3D shape is critically important for actions and interactions in the environments we inhabit, most depth cues are ambiguous. As a result, the brain requires additional information based on previous experience with the environment to infer 3D shape from depth cues. In particular, the inference of 3D shape from shading patterns (i.e., using image luminance intensity variations to derive the shape of a surface) relies on the assumption that the scene illuminant is above our heads and slightly to the left (1, 2). Understanding the illumination of a visual scene has fascinated artists and scientists for a long time (3, 4), but the neural basis of this "light from above left" preference for the interpretation of 3D shape remains largely unexplored. Although the light-from-above preference is consistent with an ecological explanation, the left bias remains entirely unexplained. More generally, the light-from-above preference provides a simple example of the way the brain represents prior knowledge and opens the door for the investigation of other types of prior knowledge related to our perception of the motion, shape, and color of objects.Previous neurophysiological and imaging studies have implicated several brain regions in the processing of shape-fromshading: primary visual cortex (V1) (5-9), areas in the caudal inferior temporal gyrus (10) and the inferior parietal sulcus (11). However, the functions mediated by these different cortical areas may differ. In particular, interpreting shape-from-shading may involve at least two different stages of processing. At the first stage, the contrast polarity of edges in the image (dark or bright) is analyzed and related to the light direction so that left and right light directions can be discriminated. At the second stage, contrast edges are grouped together to form 3D shapes so that convex and concave shapes can be dis...

International audienceThe visual system exploits prior knowledge on the world to disambiguate a scene. In particular, the inference of shape-from-shading relies on the fact that light comes from above our heads. Recent studies have helped make that assumption more precise and suggested that the preferred light source position was further biased on the left of the vertical. We investigated the generality of this result in two experiments with a different stimulus and a novel task. The stimulus was shaped like a ring (Polo Mint) divided in eight equal sectors. Depending on the assumed light source position, all sectors but one could be perceived as either convex or concave. In the first experiment, observers had to report the different shaped sector position (left or right side of the stimulus). In the second experiment, they had to classify the shape of the odd sector (convex or concave). Various amounts of blur were also applied on each stimulus. Results in both experiments confirm previous studies that observers prefer stimuli lit from the left rather than the right. We also demonstrate that left-lit stimuli give the observers a more sensitive perception of the shape of the objects. Finally, the second experiment confirms a preference for globally convex shapes, especially when the stimulus is severely blurred